collision rules

collision on all four points:
  move the player up vertically, assume they're stuck

top left and the bottom two:
  run routine for bottom two
  run routine for left two

top right and the bottom two:
  run routine for bottom two
  run routine for right two

right two:
  zero horizontal velocity
  move the player horizontally so that they're as right as they can be without colliding

left two:
  zero horizontal velocity
  move the player horizontally so that they're as left as they can be without colliding

top left only:
  move player right a bit

top right only:
  move player left a bit

top two:
  zero out the vertical velocity

bottom left, bottom right and both:
  check if we're trying to fall through a fallthroughable floor
  set a flag for being on the ground
  zero out the vertical velocity
  change the player's vertical position so that they're as far down as they can go without colliding
  if the player is pressing A and hasn't pressed A last frame, jump by setting the vertical velocity negative